Method and apparatus for inspecting printed circuit boards

ABSTRACT

A detection unit  1  for detecting an electric field or a magnetic field is placed above a circuit board  4  to be detected. A control device  3  allows the detection unit  4  to carry out detection processes while moving the detection unit  1  in a predetermined direction with an electric current being applied to the circuit board  4  so that an electric field distribution or a magnetic field distribution on the circuit board  4  is detected. Moreover, the control device  3  compares the results of the detection with reference data that has been preliminarily registered, and if there is any portion between the two pieces of data that is not coincident with each other, it is determined that the corresponding circuit board  4  is a defective circuit board. Thus, it is possible to detect any defective portion on the printed circuit board with high precision in a non-contact state to the circuit board.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a division of application Ser. No. 10/198,739, filedJul. 19, 2002 now U.S. Pat. No. 6,937,035. The entire disclosure of theabove-identified prior application is considered as being part of thedisclosure of the accompanying continuing application and is herebyincorporated by reference therein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique for inspecting a circuitboard on which various parts are packaged (hereinafter, referred to as“printed circuit board”) for any defect such as a disconnection and ashort-circuiting of a wire pattern, a defective part, a positionaldeviation, a raised state of a part, insufficient and exceedingsoldering processes and a breakdown of a part.

2. Description of the Background Art

With respect to inspections of this type, conventionally, an inspectionusing an in-circuit tester has been carried out. In this inspection, asshown in FIG. 20, after an electric current has been applied to aprinted circuit board 4, two points are selected from a wiring pattern 7and terminals 6 a, 6 b of a part 6 to be inspected, and test probes 31,32 are applied to these points so that factors, such as an electriccurrent, a voltage, an impedance or a frequency, are measured betweenthe two points with which these probes 31, 32 have been made in contact,and when the measured value is an abnormal value, it is determined thatthere is any defect between the above-mentioned two points.

In addition to this method, an inspection using a visual recognitiondevice is sometimes carried out. In this inspection, while inspectionareas are being successively set on a circuit board to be inspected,respective images are picked up so that, based upon the state of theresulting image, a determination is made as to whether or not any defectexists therein; and devices for carrying out inspections of this typeare classified into visual inspection type devices in which an inspectorvisually inspects the images so as to determine the quality of thecircuit board, and automatic inspection type devices in which, basedupon predetermined reference data, a computer determines whether or notthere is any defect.

Along with the recent developments in printed circuit boards withhigh-density, extremely fine parts and wiring patterns, it has becomedifficult to correctly put test probes onto portions to be inspected inthe case of inspections using an in-circuit tester. Moreover, when testprobes are put onto a printed circuit board having a high density, thecircuit board might be damaged due to a disconnection in the wiringpattern. In the case of a circuit board used for an apparatus requiringa high degree of safety, a redundant circuit such as a duplexed circuitis formed therein, and with respect to such a redundant circuit, testprobes are put at a position before the branch of the respectivecircuits and at a position after the branch; therefore, even when thereis any defect in one portion of the pattern, if the other pattern isnormal, the detected electrical potential by the test probes shows thesame value as the normal value, thereby failing to properly detect adefect.

Here, in the case of the inspection using a visual recognition device,when a defective portion is extremely small or a degree of a defect isvery small, the defect might be ignored. Moreover, with respect to partsthat have been packaged by using a face-down packaging method, sinceconnecting portions cannot be recognized from the face side of thecircuit board, it is impossible to carry out the detecting process byusing a visual recognition device.

SUMMARY OF THE INVENTION

The present invention has been devised to solve the above-mentionedproblems, and its objective is to detect defective portions on a printedcircuit board in a non-contact state to the circuit board, with highprecision.

When an electric current is applied to a printed circuit board, anelectric field distribution and a magnetic field distribution aregenerated in response to a circuit construction on the circuit board. Inthis case, if there is any defect such as a disconnection orshort-circuiting of a predetermined wiring pattern on the circuit boardand a breakdown of a certain part, and if the electric potential or thecurrent at this detective portion shows a value different from a normalvalue, the distribution states of the electric field and the magneticfield are varied correspondingly.

The first method of the present invention, which has been devised bytaking this principle into consideration, has an arrangement in which: acurrent is applied to a printed circuit board to be detected, andrelative positions to said printed circuit board are varied with anon-contact state to the printed circuit board being maintained so thatthe distribution state of an electric field or a magnetic field on theabove-mentioned circuit board is detected, and based upon the results ofthe detection, a determination is made as to whether or not any defectexists in the circuit board.

In order to detect the distribution state of an electric field or amagnetic field on the above-mentioned circuit board by varying relativepositions to the circuit board with a non-contact state to the printedcircuit board being maintained, for example, an electric-field detectingor magnetic-field detecting sensor is set at a position above thecircuit board, and while this sensor is being moved in the directionalong the surface of the circuit board, detecting processes are carriedout at a plurality of positions. The determination as to whether or notany defect exists is carried out by comparing the electric fielddistribution or the magnetic field distribution detected as describedabove with an optimal distribution state that is obtained when there isno defective portion. In other words, if there is any portion in thedetected distribution state that is different from the optimaldistribution state, it is possible to determine that any defect isoccurring in the corresponding different portion.

Next, with respect to a printed circuit board having no current appliedthereto, an eddy current can be generated in a closed loop on thecircuit board by using an electro-magnetic induction function. Here, inthe case when the state of the closed loop is different from the normalstate due to a disconnection or short-circuiting, etc., no eddy currentis generated, or an eddy current having a state different from thenormal state might be generated.

The second method of the present invention, which has been devised basedupon the above-mentioned principle, has an arrangement in which: amagnetic field is exerted on a printed circuit board so that an eddycurrent is generated in a closed loop formed by a conductor portionincluding patterns on the circuit board or patterns and packaged partson the circuit board, and a distribution state of a magnetic fieldgenerated by the eddy current is detected with a non-contact state tosaid printed circuit board being maintained so that it is determinedwhether or not any defect exists in the circuit board based upon theresults of the detection.

In order to generate an eddy current on a circuit board and to detectthe distribution state of a magnetic field caused by the electriccurrent with a non-contact state to the printed circuit board beingmaintained, for example, a magnetic-field detecting sensor, which has afunction for generating an induction magnetic field, is placed at aposition above the circuit board, and while relative positionalrelationship between this sensor and the circuit board is being changed,detecting processes are carried out. With respect to the determinationas to whether or not any defect exists, the same process as the firstmethod is carried out, that is, it is possible to determine whether ornot any defect exists by comparing the electric field distributiondetected as described above with an optimal distribution state that isobtained when there is no defective portion.

Here, the eddy current may be generated at an area centered on thecircuit in question; however, not limited to this method, the eddycurrent may be generated on the entire portion of the circuit board.

In the case when an ultrasonic wave is discharged to a part and a wiringpattern on a circuit board and the reflected wave is received, if thereis any defect such as a crack in the part, a positional deviation or aslated mounting of the part, and a disconnection in the wiring pattern,the degree of reflection of the ultrasonic wave becomes different fromthe normal state, resulting in a change in the level of the reflectedwave to be received.

The third method of the present invention, which has been devised basedupon the above-mentioned principle, has an arrangement in which: anultrasonic wave having a predetermined frequency is discharged to aprinted circuit board to be inspected from a position slightly apartfrom the circuit board so that the reflected wave from the circuit boardis detected, and based upon the results of the detection, adetermination is made as to whether or not any defect exists in thecircuit board.

The transmission and receipt of ultrasonic waves are carried out byusing a sensor having transmitting and receiving functions of ultrasonicwaves. In this case, this sensor is placed at a position above a circuitboard, and detection processes are carried out on a plurality ofpositions on the circuit board while varying the relative positionalrelationship between the sensor and the circuit board. Thus, it ispossible to determine whether or not any defect exists by comparing thelevel of the reflected wave thus detected at each position with anoptimal level that is obtained when there is no defective portion.

Here, in any of the first to third methods, in order to determinewhether or not any defect exists, it is preferable to preliminarilycarry out detection processes on good circuit boards without any defectunder the same conditions as the circuit board to be inspected, and toregister the results of detection as reference data so that the resultsof detection obtained upon inspection with respect to an inspectedprinted circuit board are compared with the above-mentioned referencedata.

In any of the above-mentioned methods, a circuit board is observed in anon-contact state to the circuit board by a method except for theimage-pickup method, and the determining process is carried out bydetecting a physical quantity representing whether or not any defectexists. Here, even when the defective portion is extremely small or thedegree of the defect is small, by obtaining the detected physicalquantity (a state of change in the electric field or magnetic field, ora level of a reflected wave of the ultrasonic wave) as a voltage that issufficiently great, it is possible to detect this defect with highprecision, and even when the inspection object is a circuit board withhigh density, it is possible to carry out the inspection with highprecision without causing any damage to the circuit of the circuitboard. Moreover, with respect to a circuit including a part formed by aface-down assembling process, it is possible to carry out inspectionprocesses in a non-contact state.

Moreover, in any of the above-mentioned methods, even when an areaincluding circuit patterns or parts that constitute a redundant circuitis an object of inspection in the above-mentioned printed circuit board,it is possible to detect any defect occurring in any one of the branchedcircuits, and consequently to greatly improve the inspection precision.

Here, in the first method of the above-mentioned three methods, anydefect is detected based upon a change in the electric field or magneticfield in response to an electric potential or an electric currentoccurring in the circuit upon application of a current; therefore, if,even when a defect such as short-circuiting occurs, the defect causes noabnormal value in the electrical potential and current in the defectiveportion in association with the other circuits, it becomes impossible todetect the defect. In contrast, in the second and third method, thedetermining process is carried out by using a physical quantityrepresenting a state of a wiring pattern and a part without applying anycurrent to the circuit board, it becomes possible to detect a defectiveportion that the first method would fail to detect.

Moreover, the present invention provides apparatuses that inspect aprinted circuit board by using the above-mentioned first to thirdmethods.

An apparatus in accordance with the first method is provided with: adetection unit which detects a distribution state of an electric fieldor a magnetic field on the circuit board while being set in anon-contact state with respect to the printed circuit board; a positionadjusting means for adjusting a relative positional relationship betweenthe detection unit and the circuit board; and a control means whichcontrols the position adjusting means so as to allow the detection unitto coincide with a predetermined plurality of positions on the circuitboard in succession so that a determination is made as to whether or notany defect exists in the circuit board by using detection signals of therespective positions from the detection unit.

In the case when the above-mentioned detection unit detects an electricfield, the detection unit is provided with a detection electrode whichallows an electrostatic capacity to be exerted between the circuit boardand the detection unit so as to detect this electrostatic capacity, anda shielding electrode that is used for shielding influences of anelectric field from a direction other than the circuit board from beingexerted on the detection electrode. Moreover, in the case when thedetermining process is carried out by using a magnetic field, thedetection unit is preferably provided with devices such as a magneticresistor element like an MI element and an oscillator for allowing ahigh frequency current to flow this element, and a means like a biasmagnet for generating a bias magnetic field so as to improve thesensitivity of the element.

Here, in the case when either of the electric field and magnetic fieldis detected, the detection unit is desirably arranged so that anelectric field or a magnetic field is detected at a plurality ofpositions so as to eliminate noise signals and to output a differentialsignal (that is, a change in the electric field or magnetic field in apredetermined pitch) of the detection signals obtained from therespective positions.

The above-mentioned position adjusting means may be designed so as toadjust either of the positions of the detection unit and the circuitboard. Alternatively, a mechanism for adjusting positions is placed ineach of the detection unit and the circuit board so that the positionaladjustment maybe carried out in mutually different directions, such as,in the X direction for the detection unit and in the Y direction for thecircuit board.

The control means, which controls the position adjusting operation ofthe above-mentioned position adjusting means and the detection operationof the detection unit, and acquires a detection signal from thedetection unit so as to carry out the corresponding processes, isprepared as a control circuit that is mainly constituted by a computer.Moreover, a memory, which stores inspection conditions, such as theinspection areas on a circuit board and timing in which the detectionunit is operated in the respective inspection areas, and reference dataused for determining whether or not any defect exists, may be installedin the control circuit.

An apparatus in accordance with the second method is provided with: adetection unit which applies a magnetic field onto the circuit board togenerate an eddy current in a closed loop formed by a conductor portionincluding patterns on the circuit board or patterns and packaged partson the circuit board to detect a distribution state of the magneticfield generated in the circuit board by the eddy current, while beingset in a non-contact state with respect to said printed circuit board;position adjusting means for adjusting a relative positionalrelationship between said detection unit and the circuit board; andcontrol means which controls said position adjusting means so as toallow the detection unit to coincide with a predetermined plurality ofpositions on the circuit board in succession so that a determination ismade as to whether or not any defect exists in the circuit board byusing detection signals of the respective positions from the detectionunit.

The detection unit in the apparatus having the second arrangement isconstituted by an exciting coil for exerting a high frequency magneticfield onto the circuit board and a detecting coil or a magnetic resistorelement for detecting a magnetic field generated by an eddy current.Here, the detection unit having this arrangement is preferably designedso that a magnetic field from the circuit board is detected from aplurality of positions and so that a differential signal from theresults of the respective detections is outputted.

Moreover, since a voltage, induced by an ac magnetic field, tends tohave a deviation in the phase depending on circuit constructions, theabove-mentioned detection unit is preferably provided with a circuit forselecting the phase signal of a detection signal and a circuit whichsimultaneously detects a plurality of phase signals in a separatedmanner.

The position adjusting means and the control means are designed in thesame manner as those of the apparatus of the first method. Here, in thecase when a phase signal is selected and detected in the detection unit,data, which indicates which phase signal to be selected for each of thedetection positions, may be set, and stored in a memory in thedetermination processing means.

Furthermore, an apparatus in accordance with the third method isprovided with: a detection unit which discharges a ultrasonic wavehaving a predetermined frequency to detect a reflected wave from thecircuit board while being set in a non-contact state with respect to theprinted circuit board; a position adjusting means for adjusting arelative positional relationship between said detection unit and thecircuit board; and a control means which controls the position adjustingmeans so as to allow the detection unit to coincide with a predeterminedplurality of positions on the circuit board in succession so that adetermination is made as to whether or not any defect exists in thecircuit board by using detection signals of the respective positionsfrom the detection unit.

The detection unit in the above-mentioned arrangement is provided withdevices, such as a piezoelectric element for transmitting and receivingultrasonic waves, a circuit for processing a detection output from thispiezoelectric element and a buffer member. The position adjusting meansand the control means are arranged in the same manner as those in thefirst method.

In any of the apparatuses having the above-mentioned three arrangements,the-detection unit is placed in a non-contact state to a printed circuitboard to be inspected, and detection processes are successively carriedout with respect to a plurality of positions of the circuit board in thenon-contact state to the circuit board while the relative positionalrelationship between the detection unit and the circuit board is beingvaried to determine whether or not any defective portion exists. Thisdetermining process may be carried out each time a detecting process isexecuted on each inspection subject on the circuit board; however, thedetermining process may be carried out at once after completion of thedetecting processes with respect to all the inspection subjects.

Moreover, each apparatus having any of the arrangements may be providedwith a display means for displaying the results of the determination bythe above-mentioned determination processing means, a means forexternally outputting the results of the determination and a means forstoring the results of the determination in a predetermined storingmedium (including an inner memory).

Additionally, in the case when an apparatus having the first arrangementis used, the inspecting process is carried out with a current beingapplied to the circuit board to be inspected, while in the case ofapparatuses having the second and third arrangements, the inspectingprocess is carried out with no current being applied to the circuitboard.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing that shows a schematic structure of a firstinspection apparatus in accordance with the present invention.

FIG. 2 is a drawing that shows an inner structure of a detection unit.

FIG. 3 is a drawing that shows an electrical structure of the detectionunit.

FIG. 4 is a graph that shows the relationship between an electric fielddistribution on a circuit board and the results of a detection.

FIG. 5 is a drawing that shows the relationship between the movingdirection of the detection unit and the layout of a sensor unit.

FIG. 6 is a drawing that shows another structure of the detection unit.

FIG. 7 is a drawing that shows a characteristic curve of an MI element.

FIG. 8 is a drawing that shows an electrical structure of the detectionunit shown in FIG. 6.

FIG. 9 is a drawing that shows current modes in a basic circuit for eachof the cases.

FIG. 10 is a drawing that shows current modes in a redundant circuit foreach of the cases.

FIG. 11 is a drawing that shows an example in which an eddy current isgenerated.

FIG. 12 is a drawing that shows an electrical structure of a detectionunit to be used in the second inspection apparatus.

FIG. 13 is a drawing that shows another electrical structure of adetection unit to be used in the second inspection apparatus.

FIG. 14 is a drawing that shows still another electrical structure of adetection unit to be used in the second inspection apparatus.

FIG. 15 is a drawing that shows a structure of a detection unit to beused in a third inspection apparatus.

FIG. 16 is a drawing that shows an electrical structure of the detectionunit of FIG. 15.

FIG. 17 is a drawing that shows a case in which a detection of anydefect is carried out by transmitting and receiving an ultrasonic wave.

FIG. 18 is a flow chart that shows the sequence of teaching processes.

FIG. 19 is a flow chart that shows the sequence of detecting processes.

FIG. 20 is a drawing that shows a conventional detection method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a schematic structure of an inspection apparatus inaccordance with one preferred embodiment of the present invention. Thisinspection apparatus is used for detecting defects such as adisconnection and short-circuiting of a wiring pattern, a breakdown anda defective mounting of a part and on a printed circuit board 4(hereinafter, simply referred to as “circuit board 4”) in a non-contactstate to the circuit board 4, and is constituted by a detection unit 1placed at a position above the circuit board 4, as well as a positionadjusting device 2, a control device 3, etc.

A circuit board 4 to be inspected is placed on transporting-use rails 5a, 5 b, and is transported by a driving mechanism, not shown, andpositioned to a predetermined inspection position. The above-mentionedposition adjusting device 2, which is formed by, for example, a PLC,controls a positioning mechanism (not shown) in the detection unit 1 sothat the position of the detection unit 1 is adjusted in the respectiveX and Y axis directions.

The control device 3, which controls the operations of the positionadjusting device 2 and the detection unit 1, and determines whether ornot any defect exists in the circuit board 4, is constituted by apersonal computer. The memory of this control device 3 stores the kindsand mounting positions of packaging parts, wiring patterns between theparts, inspection areas, conditions for inspection, reference data thatforms a criteria of determination in the inspection (hereinafter,referred to as “inspection-use data”) and programs used for carrying outa sequence of inspections, with respect to the circuit board 4 to beinspected. Based upon these programs and inspection-use data, afterhaving shifted the inspection unit 1 successively to each inspectionarea, the control device 3 allows the inspection unit 1 to carry out adetection process in the inspection area, while moving the detectionunit 1 in a predetermined direction (for example, X-direction).

Upon completion of the detection processes on all the inspection areas,the control device 3 compares respective detection signals with thereference data to determine whether or not there is any defectiveportion in the above-mentioned circuit board 4. The results of thedetermination are displayed on an attached monitor 3 a of the controldevice 3, and may be transmitted to another device through a datatransmission path, not shown.

Here, the above-mentioned inspection subject areas and reference dataare registered in the control device 3 through a teaching process priorto the inspection.

In the inspection device having the above-mentioned arrangement, aninspection process is carried out with a current being applied to thecircuit board 4, and the distribution state of an electric field or amagnetic field exerted on the surface of the circuit board 4 is detectedby the inspection unit 1, and the distribution state is compared withthe reference data registered through the teaching process so that adetermination is made as to whether or not there is any defectiveportion.

FIGS. 2(1) (2) show a construction of the detection unit 1 that is usedwhen the electric field distribution of the circuit board 4 is detected.The detection unit 1 of the present preferred embodiment is constitutedby two sensor units 11A, 11B that are placed in parallel with each otherin a case main body 10, and this is positioned above a part 6 and awiring pattern 7 on a circuit board 4 so as to carry out a detectionprocess. As shown in cross-sectional views of FIG. 2(2), the respectivesensor units 11A, 11B are provided with detection electrodes 14A, 14B,shielding electrodes 15A, 15B, sensor substrates 16A, 16B, etc. that arerespectively housed inside cylinder-shaped cases 13A, 13B. The detectionelectrodes 14A, 14B, which have a plate shape, are supported at lowerpositions of the cylinder cases 13A, 13B in a manner so as to maintainits upper and lower faces horizontally. Here, shielding electrodes 15A,15B are formed into a cylinder shape with its end being opened, andplaced in a manner so as to surround the detection electrodes 14A, 14Bwith the opened end facing down. In this arrangement, holes 12A, 12B(shown in FIG. 3) through which wires from the detection electrodes 14A,14B are passed are formed on the upper face of these shieldingelectrodes 15A, 15B.

FIG. 3 shows an electrical construction of the above-mentioned detectionunit 1.

The detection electrodes 14A, 14B of the respective sensor units 11A,11B are connected to ground through high resistors 19A, 19B, and theseconnections allow voltages corresponding to the electrostatic capacitiesbetween the circuit board 4 and the detection electrodes 14A, 14B to betaken out (hereinafter, this voltage is referred to as “detectionvoltage”). The respective detection voltages are inputted to adifferential amplifier 17 so that a differential signal Vio from thisdifferential amplifier 17 forms a detection signal to be inputted to thecontrol device 3. Moreover, the detection voltages obtained by therespective detection electrodes 14A, 14B are respectively given toshielding electrodes 15A, 15B of the same sensor unit through buffercircuits 18A, 18B. Thus, the shielding electrodes 15A, 15B are set tothe same electric potential as the detection electrodes 14A, 14B so thatit is possible to prevent an electrostatic capacity from generatingbetween the detection electrodes 14A, 14B and the shielding electrodes15A, 15B; therefore, only an electric field from the circuit board 4side is exerted on the detection electrodes 14A, 14B.

Here, among the circuit constructions shown in FIG. 3, those circuitslocated before the differential amplifier 17 are installed in theabove-mentioned sensor substrates 16A, 16B on the respective sensorunits 11A, 11B, except for the electrodes 14A, 14B, 15A, 15B. Moreover,the differential amplifier 17 is installed in another substrate withinthe case main body 10, together with an interface circuit, etc. to thecontrol device 3, not shown.

FIG. 4 shows the relationship between an electric field distribution onthe circuit board 4 and the results of the detection by theabove-mentioned detection unit 1; FIG. 4(1) shows an electric fielddistribution along a predetermined direction on the circuit board 4; andFIG. 4(2) shows the results (the above-mentioned differential signalVi₀) of the differential process carried out on the electric fielddistribution of FIG. 4(1).

In the respective FIGS., a distance P between respective broken linescorresponds to the distance between the above-mentioned detectionelectrodes 14A, 14B, and FIG. 4(2) indicates the differential signal Vi₀obtained when the differential process is carried out while theintermediate position between the detection electrodes 14A, 14B is madecoincident with each of the positions shown on the lateral axis of FIG.4(1) (for example, in FIG. 4(2), arrows a to d indicating theintermediate positions between the respective broken lines show thelevel of the differential signal Vi₀ obtained when the respectivedetection electrodes 14A, 14B correspond to the positions of therespective broken lines).

In accordance with the above-mentioned arrangement, the differentialsignal of the detected voltages from the respective detection electrodes14A, 14B are obtained so that the distance P between the respectivedetection electrodes 14A, 14B is allowed to function as a space filterso that noise other than the electric field caused by the constructionof the circuit board 4 can be eliminated; thus, it becomes possible todetect an electric field distribution that represents the state of apart 6 and a wiring pattern 7 on the circuit board 4.

FIG. 5 shows the relationship between the moving direction (indicated byarrow sw in the Figure) of the above-mentioned detection unit 1 and thelayouts of the sensor units 11A, 11B. In accordance with the layouts ofFIG. 5(1), the differential processes of the respective detectionvoltages are carried out in directions in accordance with the movingdirection sw, and in accordance with the layouts of FIG. 5(2), thedifferential processes thereof are carried out in directions orthogonalto the moving direction sw; however, in the inspection apparatus havingthe first construction, either of the layouts of FIGS. 5(1) and 5(2)maybe adopted. Moreover, as shown in FIG. 5(3), sensor units 11C, 11Dhaving the same constructions as the above-mentioned sensor units 11A,11B may be introduced, and by using these two sets of sensor units, thedifferential processes of the electric field distributions may becarried out in two directions, that is, a direction in accordance withthe moving direction sw, and a direction orthogonal to the movingdirection sw.

Next, FIG. 6 shows the structure of the detection unit 1 that isarranged so as to detect a magnetic field distribution of the circuitboard 4. Here, the external appearance of the detection unit 1 is thesame as the detection unit 1 used for the electric field detection, anda case main body 10 housing two sensor units 11A, 11B are supported soas to be freely shifted above an inspection position.

In the respective sensor units 11A, 11B, MI elements 101A, 101B areplaced at lower positions within cylinder cases 13A, 13B having the samestructure as that of FIG. 2(2), and bias magnets 102A, 102B, sensorsubstrates 16A, 16B, etc., are housed above these elements. The MIelements 101A, 101B have a characteristic in which the impedance isvaried in accordance with a variation in the magnetic field so that byapplying a high-frequency current to these MI elements 101A, 101B, it ispossible to obtain the variation in the impedance as a voltage.Moreover, the bias magnets 102A, 102B are used for setting a biasmagnetic field on the detection area of the above-mentioned MI elements101A, 101B.

FIG. 7 shows a change in the impedance caused by a change in themagnetic field when a current having a predetermined frequency isapplied to the MI elements. In this manner, the impedance of the MIelement is varied in a quadratic manner so that in this curve, themagnetic field H, obtained when the impedance is allowed to vary in alinear state (which refers to a change in the vicinity indicated byarrow r in the Figure), is set as a bias magnetic field, it becomespossible to improve the detection sensitivity of the MI element.

FIG. 8 shows an electrical construction of the second detection unit 1.

In the Figure, reference numeral 103 represents an oscillator thatgenerates a high frequency current of several tens of MHz. This highfrequency current is applied to the respective MI elements 101A, 101Bthrough resistors 104A, 104B, and an ac voltage, generated by the MIelement 101A, is smoothed by a rectifying circuit constituted by a diodeD_(A) and a capacitor C_(A), and taken out as a first detection voltage.In the same manner, with respect to the MI element 101B, a seconddetection voltage is taken out by a diode D_(B) and a capacitor C_(B).Further, these detection voltages are inputted to a differentialamplifier 105 so that a differential signal Vi₀ forming a detectionsignal is generated.

In this manner, in this detection unit 1 for detecting a magnetic field,magnetic fields are detected from two points apart from each other witha predetermined distance by two sensor units 11A, 11B, and by finding adifferential signal from these, noise is eliminated so that it ispossible to detect a magnetic field distribution that represents thestate of a part 6 and a wiring pattern 7 on the circuit board 4 withhigh precision.

Here, in the same manner as shown in FIG. 5, with respect to the seconddetection unit 1 also, the differential detection process of a magneticfield is carried out in either of the direction in accordance with thedrawing direction sw and the direction orthogonal to the drawingdirection sw. Moreover, it is possible to simultaneously carry out thedifferential detection processes in the two directions by using foursensor units 11A to 11D.

FIG. 9 shows respective current modes in the normal state, a statehaving a disconnection and a state having a short-circuiting withrespect to a wiring pattern and basic parts (a resistor, a diode and acapacitor). Moreover, FIG. 10 shows the current modes in the same threecases on the assumption that a redundant circuit is formed by thesewiring pattern and parts. Here, in the respective drawings, Vsrepresents a power-supply voltage, and Rs represents acurrent-regulating resistor. Moreover, in the Figure, arrows showntogether with circuits in the left column indicate directions in which acurrent I flows. (This current I may be either an ac current or a dccurrent.) Moreover, in each of the disconnection and short-circuitingcolumns, the current mode different from a normal mode is indicated by asymbol with an underline. Here, FIG. 10 assumes a case in which adisconnection or a short-circuiting occurs in the circuit shown on theupper-most level.

As shown in FIG. 9, in a normal wiring pattern and part, upon having adisconnection, a current becomes zero. In the event of such a state inwhich no current flows in a certain circuit in the circuit board 4,since the electric field distribution and magnetic field distribution ofthe circuit board 4 also vary, it is possible to detect a portion havingthe disconnection by detecting this variation. Moreover, in the event ofan over-current in the current I also, since the variation is reflectedto the electric field distribution and the magnetic field distributionof the circuit board 4, it is possible to detect a portion having ashort-circuiting by detecting the resulting variation.

Moreover, as shown in FIG. 10, with respect to a redundant circuit, evenwhen a disconnection occurs in a certain portion of the wiring patternand parts, the current I flowing through the entire circuit ismaintained in the same state as the normal state. However, since thecurrent i1 in the portion having the disconnection becomes zero, theelectric field distribution and the magnetic field distribution of thecircuit board 4 is varied due to this current change, and it is possibleto detect the portion having the disconnection by detecting this change.Moreover, in the same manner, even in the event of an over-current inthe current I flowing through a circuit due to a short-circuiting, it ispossible to detect the portion having the short-circuiting by detectingthe change in the electric field distribution and the magnetic fielddistribution caused by this current change.

Here, even in the event of a short-circuiting, no change is generated inthe current I depending on the circuit construction having theshort-circuiting (which corresponds to the current mode without anunderline in the Figure.) In this case, since hardly any change occursin the electric field distribution and the magnetic field distribution,it is impossible to carry out the detection. However, in the capacitor,since a current is always allowed to flow upon short-circuiting, itbecomes possible to determine whether or not any defect exists byprolonging the detection time or carrying out detections a plurality oftimes ((4) of FIG. 9). Moreover, as shown in (3) (4) in FIG. 10, in aredundant circuit constituted by diodes and capacitors, since thecurrents (i2, i3) flowing through parts having no short-circuitingbecome zero, it is possible to detect any defective portion based upon achange in the electric field distribution and the magnetic fielddistribution.

As described above, in accordance with the above-mentioned inspectionapparatus, since it is possible to detect any defect in a non-contactstate to the circuit board 4, no damage is caused to the circuit board 4upon detection. Even in the case when the defect is extremely small, ifthere is any change in the electric potential and the electric currentin the defective portion, it is possible to detect the defect with highprecision based upon a change in the electric field distribution and themagnetic field distribution.

The second inspection apparatus to be described next is designed so thatdefects including those which have not been detected by the firstinspection apparatus can be detected. In this apparatus, a highfrequency induction magnetic field is applied to the circuit board 4from the detection unit 1 without applying a current to the circuitboard 4 so that an eddy current is generated in a closed loop on thesubstrate 4, and by measuring a magnetic field generated on the circuitboard 4 by the eddy current, it becomes possible to determine whether ornot any defect exists.

When a high frequency induction magnetic field is exerted on the circuitboard 4 to which no current is applied, an eddy current is generated inthe circuit forming a closed loop on the circuit board 4 as shown inFIGS. 11(1) to 11(5) (in the Figures, arrows indicated by broken linesshow the directions in which the eddy current flows). When, in such aclosed loop, the state of the closed loop is changed due to adisconnection or a short-circuiting, the range in which the eddy currentflows and the strength thereof are also varied; thus, the magnetic fielddistribution caused by the eddy current is also set to a state differentfrom the normal state. Therefore, by making a check as to whether or notthe distribution state of the magnetic field caused by the eddy currentis appropriate, it is possible to determine where or not any defectexists on the circuit board 4.

FIG. 12 shows a structural example of a detection unit 1T to be used inthe second inspection apparatus. Here, since the construction of theentire inspection apparatus is the same as FIG. 1, drawings anddescriptions thereof are omitted.

The detection unit 1T of the present preferred embodiment is constitutedby a sensor main body 110 having an electromagnetic induction functionand a processing circuit 111. The sensor main body 110, which isprovided with an exciting coil 112 for applying a high frequencyinduction magnetic field to a circuit board, and a differential coil 113for detecting a magnetic flux generated by the eddy current caused bythis induction magnetic field, is designed so that, for example, theexciting coil 112 is placed on an upper position inside a case memberwith the differential coil 113 being placed at a position closer to thecircuit board surface located below. Here, reference numeral 114 in theFigure represents an oscillator which allows a high frequency electriccurrent to flow through the exciting coil 112, and reference numeral 115is a resistor located between the exciting coil 112 and the groundpotential.

The above-mentioned processing circuit 111 is constituted by a phasecircuit 116, a preamplifier 117, a multiplying circuit 118, anintegration circuit 119, etc. The above-mentioned differential coil 113removes a magnetic flux induced by the above-mentioned exciting coil 112among magnetic fluxes from the circuit board 4 so as to detect only themagnetic flux generated by the eddy current; thus, the inductionvoltage, generated in this differential coil 113, is inputted to themultiplying circuit 118 through the preamplifier 117.

The above-mentioned phase circuit 116 to which the above-mentioned highfrequency current is given through a connection path from the excitingcoil 112 to the ground is allowed to-set a signal having a phase that isthe same as the phase indicated by this current or different from thephase by a predetermined amount (hereinafter, the phase set in thisphase circuit is referred to as “set phase”.)

This set phase is given to the multiplying circuit 118 so that a signalhaving the set phase is taken out from the induction voltage of thedifferential coil 113. This detected signal is further subjected to anintegration process in the integration circuit 119, and the result ofthe process is outputted to the control device 3 as a final detectionsignal Vi₀.

Here, supposing that the current flowing through the exciting coil 112is id, the exciting magnetic field, generated by the exciting coil 112,is Φd, and the voltage inducted on the circuit board 4 by this excitedmagnetic field is ed, Φ d and ed are represented by the followingequations (1) (2). Here, Nc in equation (1) is a coefficient determinedby the inductance and the number of windings of the exciting coil 112.Φd=id·Nc  (1)ed=−dΦd/dt=−JωΦd  (2)

Moreover, in the circuit containing a resistor, a capacitor and a coil,the eddy current ie that is allowed to flow through the circuit by theabove-mentioned induced voltage ed is represented by equation (3):

$\begin{matrix}\begin{matrix}{{ie} = {{Z/e}\; d}} \\{= {{\left( {R + {J\;\omega\; L} - {{{j1}/\omega}\; c}} \right)/{- j}}\;\omega\;\Phi\; d}} \\{= {{{- j}\;{R/\omega}\;\Phi\; d} - {{L/\Phi}\; d} + {{1/\omega^{2}}c\;\Phi\; d}}}\end{matrix} & (3)\end{matrix}$

Moreover, the magnetic field Φe, generated by this eddy current ie, isrepresented by equation (4); therefore, the voltage Ee, induced by thedifferential coil 113, is represented by equation (5). Here, Ns inequation (4) is a coefficient determined by the circuit construction.

$\begin{matrix}{{\Phi\; d} = {{ie} \cdot {Ns}}} & (4) \\\begin{matrix}{{Ee} = {{- {\mathbb{d}\Phi}}\;{e/{\mathbb{d}t}}}} \\{= {{- J}\;{\omega\Phi}\; e}} \\{= {{{R \cdot {{Ns}/\Phi}}\; d} + {j\;\omega\;{L \cdot {{Ns}/\Phi}}\; d} - {j\;{1/\omega}\;{c \cdot {{Ns}/\Phi}}\; d}}} \\{\left( {{same}\mspace{14mu}{phase}\mspace{14mu}{signal}} \right)\left( {90{^\circ}\mspace{14mu}{delayed}\mspace{14mu}{signal}} \right)} \\{\left( {90{^\circ}\mspace{14mu}{advanced}\mspace{14mu}{signal}} \right)}\end{matrix} & (5)\end{matrix}$

In this manner, in the above-mentioned differential coil 113, theinduced voltage that represents the respective signals of the resistor,coil and capacitor is obtained; therefore, for example, by setting thephase advancing from the induced magnetic field by 90° in the phasecircuit 116, the induced voltage representing the signal of thecapacitor is detected, and the above-mentioned set phase is thenswitched to the phase delayed from the induced magnetic field by 90° sothat the induced voltage representing the coil signal is detected. Inthis manner, by switching the set phase, the induced voltage derivedfrom each of the kinds of parts can be detected in a separated manner.Here, the switching operation of the phase circuit 116 can be controlledby the control device 3.

FIGS. 13, 14 show another construction of the detection unit 1T.

The detection unit 1T in FIG. 13 is designed so that, in the sensor mainbody 110, a magnetic field, caused by the eddy current, is detected byusing a differential signal from a pair of MI elements 101A, 101B, inplace of the above-mentioned differential coil 112. Here, theconstruction used for obtaining a differential signal by the MI element101A, 101B is the same as that shown in FIG. 8, and the construction ofthe processing circuit 111 is the same as that shown in FIG. 12;therefore, those parts having the same functions are indicated by thesame reference numerals, and the detailed description thereof isomitted.

In the detection unit 1T shown in FIG. 14, although the construction ofthe sensor main body 110 is the same as that shown in FIG. 12, theprocessing circuit 111 is provided with three sets of circuits, each setconsisting of a phase circuit 116, a multiplying circuit 118 and anintegration circuit 119, which are placed in parallel with each other.(In the Figure, the respective combinations are represented by a, b andc in a separated manner.) Moreover, a vector processing unit 120 isplaced at the succeeding stage of the respective integration circuits119 a, 119 b and 119 c.

The same phase as that of the induction magnetic field of the excitingcoil 112, the phase having a 90° delay therefrom and the phase having a90° advance therefrom are respectively set in the respective phasecircuits 116 a, 116 b and 116 c. The induction voltage of theabove-mentioned differential coil 113 is simultaneously applied to therespective multiplying circuits 118 a, 118 b and 118 c, and the setphases in the phase circuits 116 a, 116 b and 116 c at the precedingstage are also inputted thereto. Thus, the resistor component, the coilcomponent and the capacitor component that are contained in theinduction voltage generated in the differential coil 113 aresimultaneously detected in a separate manner, and the detectedcomponents are inputted to the vector processing unit 120 through theintegration circuits 119 a, 119 b and 119 c.

The vector processing unit 120 subjects the detected components todigital processes, forms array data that have these components as therespective elements of three-dimensional vectors or data indicating thedirection and length of each vector, and outputs the resulting data as adetection signal.

In accordance with an inspection apparatus having the above-mentionedsecond construction, after the circuit board 4 has been transported toan inspection position, the inspection unit 1T is successivelypositioned at respective inspection subject areas with no current beingapplied to the circuit board 4, to carry out inspecting processes. Inthe respective inspection subject areas, while a high frequencyinduction magnetic field is being exerted on the circuit board 4, amagnetic field distribution, caused by an eddy current from eachinspection subject area, is detected, and compared with reference datathat has been preliminarily registered. Here, as described earlier, inthe case when the closed loop on the circuit board 4 has become a statedifferent from the normal state due to a defect such as a disconnectionand a short-circuiting, since a magnetic field distribution differentfrom the reference data is obtained so that it is possible to detecteven a defect that would not be reflected to an electrical potential andan electric current in the case of application of a current to thecircuit board 4, with high precision.

In the same manner as the second inspection apparatus, the thirdinspection apparatus also carries out an inspecting process with nocurrent being applied to the circuit board 4; however, in thisapparatus, in place of the above-mentioned magnetic field and electricfield, an ultrasonic wave is used for detecting any defect on thecircuit board 4.

Here, in the same manner as the second inspection apparatus, in thethird inspection apparatus also, the position adjusting device 2 and thecontrol device 3 have the same constructions as those of the first andsecond apparatuses; therefore, the following description will onlydiscuss the construction of the detection unit 1, and with respect tothe construction of the entire inspection apparatus, drawing anddescription thereof are omitted.

FIG. 15 shows a structural example of a detection unit 1S used in thethird inspection apparatus.

A detection unit 1S of the present preferred embodiment, which has acylinder-shaped case 130 as a main body, is supported above a circuitboard 4 by a position adjusting mechanism so as to freely shift. Apiezoelectric element 132 and an acoustic matching layer 133 arelaminated as upper and lower layers at a lower position inside the case,and a sensor substrate 134 in which a processing circuit which will bedescribed below is installed, electrode terminals 135 a, 135 b thatrelay this sensor substrate 134 to the piezoelectric element 132, etc.are assembled therein.

The above-mentioned piezoelectric element 132 is allowed to vibrate uponreceipt of a driving current from a high frequency oscillator, notshown, to generate an ultrasonic wave, and receives a reflected wavefrom the circuit board 4 so as to generate a voltage corresponding tothe level of the received wave. The acoustic matching layer 133 subjectsthe ultrasonic wave generated by the piezoelectric element 132 to amatching process with respect to the vibration characteristic, andtransmits this to the air. Here, in the above-mentioned case 130, thepiezoelectric element 132 is surrounded by a resin buffer member 136 onits sides and its upper portion, with the above-mentioned electrodeterminals 135 a, 135 b being supported by a terminal holder 137 at aposition above this.

FIG. 16 shows a structural example of a processing circuit 140 whichprocesses an output (indicated by “piezoelectric output ΔVi” in thedrawing) from the piezoelectric element 132.

This processing circuit 140, which acquires the piezoelectric output ΔViin synchronized timing with the receipt of the reflected wave from thecircuit board 4, and carries out an amplifying process on this, ismainly constituted by an operation amplifier 141. Here, the operationamplifier 141 is designed to a positive feed-back type so as to adjustthe offset level of the detected voltage in the case when no reflectedwave is received; thus, feed-back resistors 142, 143 and voltage-settingresistors 145, 146 are connected thereto, and a hysteresis adjustingresistor 144 is placed between the input and output on the + side.

A pre-amplifier 147 and a gate circuit 148 are placed at the precedingstage of the operation amplifier 141, and a buffer circuit 149 is placedat the succeeding stage thereof. A control signal has been given to thegate circuit 148 by a control circuit, not shown, or the above-mentionedcontrol device 3, and after a piezoelectric output ΔVi, given while thegate circuit 148 is open, has been acquired by the operation amplifier141, and amplified therein, the signal Vi₀, given through the buffercircuit 149 located at the succeeding stage, is inputted to the controldevice 3 as a detection signal. Here, the above-mentioned control signalis set so that, after the gate has been closed until a reflected wavehas been received from the circuit board 4 since the transmission of anultrasonic wave by the piezoelectric element 132, the gate is opened fora predetermined time.

FIG. 17 shows cases in which transmitting and receiving processes ofultrasonic waves make it possible to detect any defect.

FIG. 17(1) shows a detection process for a part 6 having a normallypackaged state in which the level of an ultrasonic wave to betransmitted to the circuit board 4 (hereinafter, referred to as“transmission wave level”) is represented by Ft, and the level of areflected wave received from this part (hereinafter, referred to as“received wave level”) is represented by Fr. Here, FIGS. 17(2) to 17(4)show detection processes that are applied when any defect occurs in thepart 6 or in the mounted state thereof; and these Figures show states onthe assumption that, with respect to the transmission wave having thesame transmission level Ft as that shown in FIG. 17(1), reflected waveshaving respective received wave levels of Fr1, Fr2, Fr3 are obtained.Here, in the Figures, reference numeral 8 represents a soldered portion.

FIG. 17(2) shows detection processes that are carried out under thecondition that a crack occurs in the part 6 or a disconnection occursinside the part. In this case, it is considered that, since one portionof the transmission wave is absorbed inside the part 6, the receivedwave level Fr1 becomes smaller than the level Fr in the normal state.

FIG. 17(3) shows detection processes that are carried out when there isa defect in soldering such as a hollow soldered portion due to excessivesoldering and an insufficiently soldered portion and the subsequent riseof the part. When, an ultrasonic wave is applied to a part 6 having sucha defective soldered portion, local vibration occurs in the hollowportion of the soldered portion 8 and the part 6, with the result thatone portion of the transmission wave is absorbed by a wiring pattern 7and the part 6, or reflected in a direction different from the normalstate. Therefore, in this case also, it is considered that the receivedwave level Fr2 becomes smaller than the level Fr in the normal state.

FIG. 17(4) shows detection processes that are carried out when a crackor a disconnection occurs in the wiring pattern 7. When an ultrasonicwave is applied to a portion having such a defect, local vibrationoccurs in the disconnected wiring pattern 7 and the part 6, with theresult that one portion of the transmission wave is absorbed by a wiringpattern 7 and the part 6, or reflected in a direction different from thenormal state. Therefore, in this case also, it is considered that thereceived wave level Fr3 becomes smaller than the level Fr in the normalstate, in the same manner as the cases of FIGS. 17(2), 17(3).

In this manner, in the event of any defect in the circuit board 4, evenwhen an ultrasonic wave having the same level Ft as that in the normalstate is applied thereto, the level of a reflected wave to the detectionunit 1 is varied due to a variation in the reflection state of theultrasonic wave. Therefore, with the conditions of the transmission wavebeing maintained constant, the reflected wave from each detectionsubject area is received, and by comparing the level of the reflectedwave with reference data that has been preliminarily registered, it ispossible to detect whether or not any defect exists with high precision.

Here, since the inspection subject area on the circuit board 4 isextremely small, the detection unit 1 to be used in the third detectionapparatus is preferably designed so that it can emit an ultrasonic wavethat exhibits abrupt rises locally.

Moreover, in an arrangement in which an ultrasonic wave having aspecific frequency is transmitted to the inspection subject area so asto give an impulse and the frequency spectrum of the reflected wave isobserved, the reflected wave comes to have a difference in its frequencyspectrum depending on the cases in which the inspection subject area isgood and in which there is any defect therein. Therefore, an attempt ismade so as to variably set the frequency of the above-mentionedultrasonic wave within a predetermined frequency range, and in thisarrangement, an ultrasonic wave having a certain set frequency istransmitted, and by extracting a reflected wave component having atuning frequency, it becomes possible to determine whether or not anydefect exists. Moreover, when an attempt is made so as to variably setthe level of the transmission wave, the level of the transmission wavecan be adjusted to an optimal level for receiving the reflected wavehaving the above-mentioned tuning frequency, thereby making it possibleto carry out an inspection with high precision.

In any of the above-mentioned first and third inspection apparatuses,prior to the inspection, a teaching process for registering referencedata used for determining the inspection subject area and whether or notany defect exists is carried out, and the inspecting process is thenexecuted. The main sequences of these teaching processes and theinspection processes are in common with any of the inspectionapparatuses; therefore, the following description will briefly discussthe sequences of these common processes. In the following explanation,with respect to the inspection units 1, 1S, 1T of the respectiveinspection apparatuses, these are generally referred to as “inspectionunit 1” for convenience of explanation.

FIG. 18 shows a sequence of teaching processes (hereinafter, therespective steps are indicated as “ST”).

The teaching processes are carried out by using a good circuit boardwithout any defective portion (hereinafter, this is referred to as“reference circuit board”). First, at ST1, a reference circuit board isplaced onto an inspection position. Here, in the case when the firstinspection apparatus is used, a current is applied to the circuit boardat this step ST1; however, in the case when the second and thirdinspection apparatuses are used, the sequence proceeds to the followingsteps without applying a current to the circuit board.

At the next step ST2, the operator is allowed to set an inspectionsubject area. After this setting has been made, the setting position,the size, etc. of the inspection subject area are registered in amemory, and the sequence proceeds to the next step ST3.

Here, at ST2, by using a method in which map information of the circuitboard, preliminarily stored, is displayed so as to allow the operator tospecify the inspection subject area on the map, it is possible to easilyacquire setting data of the inspection subject area. Alternatively,default setting data is preliminarily registered as the inspectionsubject area, and at ST2, this setting data is displayed so as to allowthe operator to confirm the data, and the setting data may be revisedonly when an instruction for a revision is given.

Here, the inspection subject area may be set to a desired size.Moreover, in the case when a redundant circuit has been set, theinspection subject area may be set in a manner so as to include theentire portion of this redundant circuit, or the inspection subject areamay be set on each branched circuit basis.

When the inspection subject area has been set in this manner, at ST3,the detection unit 1 is shifted to the first inspection subject area.Then, within this inspection subject area, the detection unit 1 isallowed to carry out detecting processes at a plurality of positionswhile being moved therein, and a detection signal indicating the resultsof the detection is acquired, and registered in a memory as referencedata (ST 4, 5). Here, this acquiring process of this detection signalmay be carried out at positions specified by the operator within theinspection subject area, or detection processes may be carried outsuccessively every predetermined interval.

Moreover, in the case when the second inspection apparatus is used, atthe above-mentioned ST4, in accordance with kinds of parts within theinspection subject area, the setting phase to the phase circuit 116 andthe order of switching of the set phase are determined, and these may beregistered in the memory as the inspection conditions. Moreover, in thecase when the third inspection apparatus is used, at ST4, the timing inwhich the above-mentioned gate circuit 148 is opened and closed isdetermined, and these may be registered in the memory as inspectionconditions.

Upon completion of the processes within the inspection subject area inthis manner, “YES” is given at ST5, and after the sequence has returnedfrom ST6 to ST3, the same processes are executed on the next inspectionsubject area.

In the same manner as described above, the detection unit 1 is shiftedto the respective inspection subject area and the detection process iscarried out while the detection unit 1 is being moved within the area sothat the reference data is accumulated successively. Here, therespective reference data is desirably set to have a data structure inwhich in addition to the value of the detection signal obtained from thedetection unit 1, the coordinate position of the detection unit 1 uponacquiring the detection signal is associated with the value.

Upon completion of the processes on all the inspection subject areas,“YES” is given at ST6, thereby completing the teaching processes.

FIG. 19 shows the processing sequence at the time of the inspection.Here, in FIG. 19, the steps of the respective processes are indicated bysymbols of ST11 and thereafter so that these are not mixed with theaforementioned teaching sequence.

First, at Step ST11, a circuit board 4 to be inspected is shifted andpositioned onto an inspection position. Here, in the case when the firstinspection apparatus is used, a current is applied to the circuit board4; however, in the case when the second and third inspection apparatusesare used, the following steps are carried out without applying a currentto the circuit board 4.

At the next step ST12, the inspection subject area registered in theabove-mentioned memory is read out, and at ST13, the detection unit 1 isshifted to this inspection subject area.

Next, a detection process is carried out on each position correspondingto the above-mentioned reference data while the detection unit 1 isbeing moved within the above-mentioned inspection subject area (ST14,15). The detection signal obtained through this process is stored in thememory as inspection data.

Upon completion of the processes within the inspection subject area,“YES” is given at ST15, and the sequence proceeds to ST16, where theinspection data is compared with the reference data. Here, in thiscomparing process, for example, a difference is found between theinspection data and the reference data, and when the difference valueexceeds a predetermined threshold value, it is determined that the twovalues are not coincident with each other.

When there is any inspection data that is not coincident with thereference data in the above-mentioned comparing process, “YES” is givenat the next step ST17, and the sequence proceeds to ST18 where theposition at which this inspection data is obtained, the degree ofnon-coincidence, etc. are stored in the memory as defect information.Thereafter, the sequence returns from ST19 to ST12, and the sameprocesses are executed on the next inspection subject area.

Upon completion of processes with respect to the entire inspectionsubject areas in this manner, “YES” is given at ST19, and the sequenceproceeds to ST20 where a determination is made as to whether or not anydetect information has been stored in the memory. Here, if no defectinformation has been stored, the sequence proceeds to ST21 in which thecircuit board 4 that has been inspected is determined as a good circuitboard. In contrast, if any defect information has been stored, thesequence proceeds to ST22 in which the circuit board 4 that has beeninspected is determined as a defective circuit board.

Upon completion of determination as to a good or defective circuitboard, the sequence proceeds to ST23, and the result of thedetermination is outputted. Here, in the case of determination as adefective circuit board at ST22, it is preferable to output detailedinformation including the defect information and the position of theinspection subject area, together with the results of the determination.

As described above, in the present invention, a printed circuit board isobserved in a non-contact state to the circuit board and the determiningprocess is carried out by detecting a physical quantity representingwhether or not any defect exists; therefore, even when the inspectionsubject is a circuit board with high density, it is possible to carryout the inspection with high precision without causing any damage to thecircuit of the circuit board. Moreover, with respect to a circuitincluding a part formed by a face-down packaging process, it is possibleto carry out inspection processes in a non-contact state.

In particular, in the second and third methods of the present invention,a magnetic field or an ultrasonic wave is applied to a circuit boardwith no current being applied thereto, and the determining process iscarried out by using a physical quantity (reflection wave of themagnetic field or the ultrasonic wave caused by an eddy current)generated by the application; therefore, it becomes possible to detectany defect that has not been detected since no change occurs in theelectrical potential and electric current in the inspection with acurrent being applied to the circuit board; thus, it becomes possible togreatly improve the detection precision.

1. A method for inspecting a printed circuit board comprising the stepsof: applying a magnetic field onto said printed circuit board using amagnetic field inducing structure without physically contacting theprinted circuit board to generate an eddy current in a closed loopformed by a conductor portion including patterns on said printed circuitboard or patterns and parts on said printed circuit board; detecting adistribution state of a magnetic field generated by the eddy currentwith a non-contact state to said printed circuit board being maintainedusing a magnetic field sensing structure, the magnetic field sensingstructure and magnetic field inducing structure being integral; anddetermining whether or not any defect exists in said printed circuitboard based upon the results of said detecting.
 2. The method forinspecting a printed circuit board according to claim 1, wherein saidprinted circuit board has an area including a wiring pattern or partsconstituting at least redundant circuit as an object for inspection. 3.The method for inspecting a printed circuit board according to claim 1,wherein: a detecting process is preliminarily carried out on a goodcircuit board without any defect under the same conditions as a circuitboard that is an object for inspection so as to register the results ofthe inspection as reference data, and upon inspection, a determinationis made as to whether or not any defect exists by comparing the resultsof inspection obtained with respect to an object for inspection withsaid reference data.
 4. An apparatus for inspecting a printed circuitboard comprising: a detection unit which applies a magnetic field ontosaid printed circuit board using a magnetic field inducing structurewithout physically contacting the printed circuit board to generate aneddy current in a closed loop formed by a conductor portion includingpatterns on said printed circuit board or patterns and packaged parts onsaid printed circuit board to detect a distribution state of a magneticfield generated in said printed circuit board by the eddy current usinga magnetic field sensing structure, the magnetic field sensing structureand magnetic field inducing structure being integral, while being set ina non-contact state with respect to said printed circuit board; positionadjusting means for adjusting a relative positional relationship betweensaid detection unit and said printed circuit board; and control meanswhich controls said position adjusting means so as to allow thedetection unit to coincide with a predetermined plurality of positionson said printed circuit board in succession so that a determination ismade as to whether or not any defect exists in said printed circuitboard by using detection signals of the respective positions from thedetection unit.